Emergency thread supply devices are known in the art to employ an electromagnetic thread breakage detector coupled to a mechanical arm. The arm is engaged by yarn withdrawn from the spinning unit by feed rollers. When a break in the yarn occurs, the arm swings from a normal operating position to an emergency operating position, triggering a reed relay in the process. The relay disengages the electromagnetic coupling that drives the fiber supply.
The electric thread breakage circuit is usually protected from short circuit by a thermal fuse, but there are circumstances when this safeguard is insufficient. If the thread breakage circuit malfunctions to the extent that the reed relay is not closed, the device will fail to disengage the fiber supply. This in turn will cause the rapid and dangerous accumulation of highly flammable fiber. In such a case, the fiber is continuously provided by the electromagnetic coupling but no fiber is consumed by the spinning machine because of the essentially undetected break.
West German published application DE-OS No. 2,214,066 discloses a safety system comprising a thermal fuse connected to the electric circuit of an electromagnetic fiber supply coupling. A two-arm lever is associate with the fuse such that the fiber supply is disengaged even when the breakage detector fails or short circuits. The thermal fuse in turn is controlled by the transfer of heat from the spinning rotor interior via a heat conductor. This system suffers from a number of disadvantages. It is complicated and often unreliable in practice, because a relatively large swing of the two-arm lever is required to trigger the interrupt, without regard for the potential combustion of accumulated thread. Heat is conducted to the thermal fuse slowly, and the system therefore fails to react quickly in the event of a fire in the choked spinning rotor.
Another thermal fuse safety device is disclosed in West German published application DE-OS No. 2,212,106. In this arrangement, a pair of resistors are connected to a fiber supply current source in series with a switch element and a thermal fuse. A thermal sensor connected to one of the resistors is disposed within the spinning mechanism. The sensor responds to the rise in temperature caused by friction of the accumulated fiber, and this disconnects the resistor and interrupts the entire circuit. However, the system remains relatively complicated and responds too slowly.